This invention relates to a cesium-containing optical glass which is especially suitable for light converging-type lenses.
Cesium-containing optical glass is known from Japanese Patent Publication No. 21594/76 (corresponding to U.S. Pat. No. 3,827,785). This patent document describes a light converging-type lens having the composition of 2-50 mole% Cs.sub.2 O, 30-98 mole% SiO.sub.2 and 0-30 mole% B.sub.2 O.sub.3, the sum of SiO.sub.2 +B.sub.2 O.sub.3 being 50-98 mole%, at its central axis, with the concentration of cesium gradually decreasing outwardly from its central axis and the concentration of an alkali metal other than cesium gradually increasing in the same direction. It also states that this light converging-type lens is produced by molding a glass having the above composition at its central axis into a rod, and treating the glass rod in a molten salt containing potassium such as potassium nitrate at a temperature slightly higher (e.g., 560.degree. C.) then the yield point (e.g., 500.degree. C.) of the glass for a long period of, for example 120 hours, thereby exchanging the cesium ion with a potassium ion.
The light converging-type lens so produced, however, has defects in that because of the said ion exchange, the surface of the lens is corroded by the molten salt, its strength is reduced and breakage of the lens occurs during the manufacturing process, or the diameter of the lens becomes nonuniform and the corroded portion of the glass cannot be effectively used as a lens.
There is also known a glass composition suitable for production of a light transmitting glass body, which is composed mainly of SiO.sub.2 (35-60 mole%), B.sub.2 O.sub.3 (30-45 mole%) and Cs.sub.2 O (8-25 mole%) and in which the cesium ion is substituted by an alkali metal ion from an external source to provide a refractive index continously decreasing from its center toward its peripheral surface (see Japanese Laid-Open Patent Publication No. 87106/75).
This cesium-containing glass is the same as the glass disclosed in the first-cited prior art reference in that it contains SiO.sub.2, B.sub.2 O.sub.3 and Cs.sub.2 O.